1. Field of the Invention
The present invention relates generally to improvements in apparatus for balancing rotating objects, and, more particularly, to improvements in the balancing of disc drive disc assemblies.
2. Brief Description of the Prior Art
In a disc drive, computer files are stored along concentric data tracks that are defined in magnetizable surface coatings of rotatable discs. More particularly, files are saved as sequences of data track segments, or sectors, that are each magnetized in a pattern that reflects the contents of a portion of the file and the file can subsequently be retrieved by sensing the magnetic field that these segments generate adjacent the disc surfaces.
To implement this mode of storage and retrieval, the discs are rotated during operation of the disc drive and read/write heads that "fly" over the disc surfaces are aligned with selected data tracks to write the file as the magnetic medium passes the head, via currents passed through the head, or to read the file via emfs induced in the head by passage of so-called flux transitions along the data track. The heads are mounted on an actuator that is positioned by a servo system, operating from signals induced in at least one of the heads, that generates actuator correction signals from servo patterns that were written to one or more disc surfaces at the time the disc drive was manufactured. By using such a servo system to position the actuator and, consequently, the heads, the data tracks dan be very closely spaced to result in a large data storage capacity for the disc drive.
However, as will be clear to those of skill in the art, the more closely the data tracks are spaced, the more difficult it becomes to follow a selected data track at which a file is to be stored or to which a file has been previously stored. More particularly, mechanical vibration of the disc drive can, if sufficiently severe, interfere with track following by the read/write heads for track spacings that can be readily achieved using current head technology. As a result, unless vibration is controlled, track densities and, consequently, data storage capacities are artificially limited. Further, vibration gives rise to noise that is unwelcome in the environments in which computers are used.
A source of vibration that is inherent to the disc drive is vibration arising from the rotation of the discs on which files are stored. The discs have the general form of circular platters, having central, circular apertures, and they are mounted on the case of a disc drive by clamping them to a cylindrical hub which passes through the apertures and is itself mounted on the disc drive case for rotation about the cylinder axis of the hub. The discs are separated by spacers, which have a toroidal form to fit over the hub between the discs and the entire disc assembly, including the discs, the hub, and the spacers, is clamped together by a clamp ring that is mounted on one end of the hub to bear against one end of the disc stack, the other end of which is supported by a circular flange on the end of the hub.
As will be clear from the above description of the disc assembly, the disc assembly is, at least in principle, cylindrically symmetrical with respect to the rotation axis of the assembly; i.e., the axis of the hub. Consequently, and again in principle, the center of mass of the assembly will be located on the rotation axis and the assembly will be statically balanced for rotation on the disc drive case. In practice, a number of factors give rise to an imbalance of the disc assembly. For example, small variations occur in disc thickness with location on the disc surface; the discs are not perfectly circular; and the central mounting apertures are not perfectly concentric with the outer edges of the discs. Similarly, the hub itself will generally not be perfectly balanced nor, in general, will the discs be perfectly positioned on the hub.
While, these factors are each small and, indeed, while steps are taken in the manufacture of a disc drive to minimize the effects of these factors, they nevertheless add up to cause an imbalance of the disc assembly that is sufficient, unless compensated, to cause vibration of the disc drive that will interfere with track following and cause noise problems at the track densities and rotation speeds that are presently in use and the even higher densities and speeds contemplated for the future. Accordingly, it has become standard practice to balance the disc assembly at the time of manufacture of the assembly.
However, conventional methods for balancing the disc assembly present problems in their own right. Thus, for example, one approach to balancing a disc assembly that has been used in the past is to place pieces of adhesive backed lead foil in strategic locations on the assembly. One problem with this approach is that the foil may come loose after the disc drive has been in operation over an extended period of time because of aging of the adhesive. A second problem is that the adhesive forms a source of large organic molecules which can adhere to, and build up on, the heads to interfere with the "flying" of the heads above the disc surface. With smaller flying heights that are expected in future disc drives, this latter problem is by no means inconsequential.
A second approach that has been used in the past is to drill a series of symmetrically spaced holes in one end of the hub, tap them and use set screws of various lengths mounted in the holes to effect the final balance. This approach suffers from several disadvantages. Because of the number of parts involved, both the amount of machining required to implement the approach and the time required for the subsequent installation of the screws undesirably increases the cost of the disc drive. Moreover, the tapped holes can trap particulate matter that can later be dislodged and adhere to a disc surface to interfere with the flight of heads adjacent the surface. As is well known in the art, the heights at which heads fly over disc surfaces are measured in microinches so that even a very small particle on a disc surface acts as a large obstacle to a head. Consequently, the particle represents a surface flaw that prevents portions of the surface from being used for the storage of data.
As a result, while it is known to balance the disc assemblies of disc drives and while a variety of techniques have been developed for this purpose, the prior art balancing techniques leave much to be desired. Moreover, as rotation speeds increase, head flying heights decrease and track densities increase, it can be expected that the prior art disc assembly balancing techniques will become even less suited to the basic purpose for which they were developed.